The constantly meshed type multi-stage transmission has a configuration in which one of the drive gears and the driven gears are fixed on a gear shaft, while the other of the drive gears and the driven gears are rotatably supported on another gear shaft, and in which one of the rotatably supported gears is selected to be engaged with the other gear shaft by an engaging means, to make a gear shift.
There are patent publications disclosing a multi-stage transmission in which driven gears are rotatably supported on a driven gear shaft and engaging means are provided to make engagement and disengagement between each of the driven gears and the driven gear shaft (refer to Patent Document 1, for example).
[Patent Document 1]
Japanese Patent Publication No. 45-35687
The engaging means used in the constantly meshed type multi-stage transmission disclosed in Patent Document 1 has such a structure that a tubular body is provided which is formed with axially extending grooves each having a widely cut hole extending in the circumferential direction at a middle portion of each groove, the tubular body is externally fitted over a driven gear shaft so as to be movable in the axial direction, and a plurality of driven gears are rotatably supported on the tubular body.
The driven gear shaft has in the outer surface thereof flat cutouts at axial positions corresponding to the driven gears. Rollers are put respectively on each of the flat cutouts of the driven gear shaft, and are loosely fitted in the grooves and the widely cut holes of the tubular body, the rollers in each of the grooves being arranged in a row.
Regarding the driven gears rotated while being constantly meshed with the drive gears, the rollers in the grooves do not make contact with the inner peripheral surfaces of the driven gears, so that rotation is not transmitted from these driven gears, and only the rollers in the widely cut hole makes contact with and are engaged with the inner peripheral surface of the relevant driven gear, so that rotation is transmitted from the relevant driven gear.
In other words, each of the driven gears at a position where the widely cut hole is located transmits rotation to the driven gear shaft, so that gear shift can be achieved by moving the tubular body in the axial direction.